This invention is directed to a method for achieving windshield sealant adhesion over a basecoat/clearcoat finish in which the clearcoat composition comprises a carbamate polymer or oligomer. In particular, this invention is directed to a method for obtaining primerless windshield sealant adhesion over a carbamate clearcoat, especially during automobile or truck assembly operations.
In order to protect and preserve the aesthetic qualities of the finish on an automobile or other vehicle, it is generally known to provide a clear (unpigmented or slightly pigmented) topcoat over a colored (pigmented) basecoat, so that the basecoat remains unaffected even on prolonged exposure to the environment or weathering. This is referred to as a basecoat/clearcoat finish. It is also generally known that carbamate polymers alongside melamine crosslinking agents provide coatings with improved chemical or etch resistance, due to the formation of desirable tertiary urethane linkages in the coating upon cure. Exemplary of prior patents disclosing carbamate polymers for coatings are U.S. Pat. No. 6,451,930 and U.S. Pat. No. 6,239,212.
Commercialization of carbamate finishes have been hindered by several significant or even critical technical hurdles. For example, a commercially practical finish, among other requirements, must have adequate adhesion to windshield sealants or adhesives, which are typically moisture-cure adhesives containing isocyanate groups, such as those described in U.S. Pat. No. 5,852,137.
Typically when a windshield is affixed to the body of a vehicle which has already been painted, a sealant material is used to attach the windshield to the body. However, many of the commonly available windshield adhesives do not adhere well to clearcoats that contain carbamate groups. One solution to the problem of failure of windshield sealants to adhere to carbamate containing clearcoats is to prime the clearcoat with a urethane primer wherever the adhesive is to be applied. Although effective, this method adds an additional step to the process of adhering a windshield to the vehicle body. Another solution to the problem is to add hydrolyzable silane additives to the coating to enable cross-linking between the active silane groups on the surface of the cured clearcoat and active silane groups in the moisture-cure windshield sealant to achieve windshield sealant adhesion. However, weak acid catalysts, such as phenyl acid phosphates, that are commonly used nowadays in commercial basecoats for better appearance tend to diffuse or migrate from the basecoat into the clearcoat and destroy the activity of the silane in the clearcoat.
Continuing effort has thus been directed to the achievement of primerless windshield sealant adhesion over a basecoat/carbamate clearcoat finish, while also meeting today's performance requirements, such as high gloss, DOI (distinctness of image) and low orange peel, etch, scratch and mar resistance, adhesion to additional in-line or end-of-line repair coatings, and low VOC (volatile organic content) emission requirements.